This invention relates to metal detectors, and specifically to detectors that discriminate between ferrous and non-ferrous metals.
Discriminating metal detectors are well known. More than 80 patents claim to have solved the problems inherent in such detectors, but the improvements in the art have been introduced in such small increments that a level of performance that satisfies the demands of the market place has not yet been attained.
Discrimination is said to be functional “if ground permits” or “if the signal is not too weak”. Graphical representations of the discrimination capability show that it is operative to a target depth that amounts to less than a quarter of the maximal detection depth.
The difficulties encountered by the state-of-the-art detectors stem from the fact that the resistive and reactive components of the signal intercepted by the receiver coil are intermingled and the various methods used to separate them are very complex and the results obtained are not good enough to achieve reliable discrimination against ferrous metals.
Instead of attempting to analyze the complex target signals, the present invention separates them into their components at their very source, by using a unique coil-current wave form.
A partial separation of the received signals into in-phase and quadrature signals, has been achieved in prior art.
Karbowski, in U.S. Pat. No. 4,906,973 describes such a system. However, his system fails to differentiate between signals of different origins, the effect of which will be discussed below in further detail.
The above-cited patent senses the reactive component only when a target is present, as determined by the presence of a resistive signal—a feature in common with the present patent. The purpose for doing that is materially different in the present patent, however. Karbowski's purpose is to differentiate between a human body and a metallic object; in the present patent, the object is to differentiate between ferrous and nonferrous targets.
The purpose for initiating the reactive-component sensing when the resistive component is present, is that the reactive components originating in the background must be nulled out immediately prior to the sensing of reactive signals caused by the target. This must be accomplished dynamically, as the environment and the coil assembly alignment change. Such a dynamic alignment requires an electronic feedback loop. Karbowski describes only mechanical means for nulling the detector.
Other means for nulling or “balancing” a detector are also used in prior-art hand-held detectors. For example, a synchronous demodulator, with the signal gating pulse straddling the zero crossing point, as shown in FIG. 1, yields one output signal polarity for ferrous targets and the opposite polarity for non-ferrous targets. This is true, when the coil system has been balanced for a signal that contains both resistive and reactive components. In the balanced condition, discrimination between ferrous and non-ferrous targets is possible, however, presence of magnetic minerals in the ground often causes misidentification of a target. A change in the reactive component of the signal causes a phase shift, as shown in FIG. 2. The detector can not determine whether this is caused by a target, or a change in the soil.
Attempts to separate the target signal from the background signal have resulted in the design of detectors which use multiple operating frequencies and process the resulting signals with extremely complicated circuits. The drawbacks of such methods are obvious when one considers the problems of alignment, power consumption and drift of complex systems, under the temperature and humidity conditions that metal detectors are expected to endure.
The objective has been to make use of the fact that the ratios between the resistive and reactive components of the target signal vary with the nature of the target and the frequency used. Thus, it is theoretically possible to identify a target by comparing the received signal to memorized “signatures” of desirable and undesirable targets.
Owing to the fact that some undesirable objects like “hot rocks” present an infinite variety of signatures, this type of system represents only a partial solution to the problem. The fact that many signatures of desirable and undesirable targets overlap is another shortcoming of such a system.
To overcome the limitations of sine-wave systems, pulse-induction systems have been developed. In such systems, the target signal is sampled at a time when the primary field is absent, thus eliminating signals owing to mutual inductance between the receiver and transmitter coils. This expedient greatly reduces a detector's sensitivity to magnetic minerals in the soil, but it also eliminates the detector's ability to discriminate between ferrous and non-ferrous targets. Such “all-metal” detectors are used mostly in industrial applications, where any metal contamination must be detected and removed from a product.
A certain level of in-phase signal is present at nearly all times, owing to soil conductivity or the presence of minerals that exhibit magnetic viscosity or energy absorption and release effects.
This background signal must be eliminated so that it does not affect the threshold at which a target is detected.
The simplest method of prior art uses capacitive coupling between the demodulator and the level sensor, to eliminate the DC component of the signal.
Such detectors are known as “motion detectors”. Capacitive coupling of the signal eliminates one problem but introduces a new one.
A transient, negative-going excursion of the signal, caused by a void or an inert rock in the ground produces a rebound, which results in a positive signal. A human ear may be able to learn to recognize the characteristic sound produced by this phenomenon, but a level sensor can not. An industrially usable detector must issue a non-ambiguous signal such as a logic pulse, to stop a conveyor belt or to operate automatic target processing machinery.
More sophisticated detectors use what is known as a “self-adjusting threshold”, which periodically restores the background signal to zero. In some instruments, the rate at which this occurs is variable, to make it possible to strike a reasonable balance between instability and loss of sensitivity.
Owing to the possibility of cancelling out a target, this method is not usable in detectors monitoring a conveyor belt, or in other sensitive applications.
In the present invention, the background signal is eliminated by a method which is not subject to the above limitations.
The degree of mineralization of the ground may change during the very time the target signal is being acquired, thereby affecting the target identification process. This problem is not addressed in prior-art-detectors.
In the present invention, the background change is assessed by evaluating the background immediately before and after the target signal is acquired. By this means, the effect of the changing background is eliminated.
A similar problem arises when several targets are located in close proximity and one of them exhibits undesirable characteristics. In prior-art detectors, signals from the undesirable target cause all the targets to be misidentified.
The present invention comprises a means to detect the presence of multiple targets, lessening the probability of a good target being masked by an undesirable one.
Owing to the above-described problems with state-of-the-art metal detectors, there is a need for an improved detector, particularly for use in gold mines, recycling facilities, security applications, food processing and for land mine detection.
Even detectors used by hobbyists benefit from the ability to discriminate between worthless and valuable targets, without the need to interpret ambiguous visual and auditory cues.
The failure of prior-art technology to solve the problems associated with target identification can be largely attributed to the use of techniques which do not identify and counteract the various signals that are elicited when a magnetic flux pulse penetrates the ground and the signals generated owing to mutual inductance between the transmitter and receiver coils.
In contrast, the present invention detects the various signals engendered by the coil pulse and provides the means to nullify the ones that impair detector performance.
Theory
To facilitate the understanding of the operation of the present invention, the underlying physical principles are outlined below.
FIG. 3-A shows the shape of the coil current. In accordance with Faraday's Law of Electromagnetic Induction, a changing magnetic field produces a voltage which is proportional to the time-derivative of the magnetic field.
In the case of a linearly changing field, the induced voltage is a steady DC level, as shown by trace 312, in FIG. 3-B.
When such a voltage is generated in a conductive object, a current results. The magnitude of the current is initially zero, and it gradually attains a maximal value which is determined by the induced voltage and the resistance in the current path. This current is generally know as a Foucault current, or more popularly, as an eddy current.
The speed with which the current attains its maximal value depends on the ratio between the inductance and the resistance of the current path. This quantity has been given a the name “time constant”, according to the formula:T=L/R where T is the time constant, L is the inductance and R is the resistance.
During the build-up phase, the eddy current follows an exponential path, which is defined by the equation:I=I.sub.max×(1−e.sup.(−t/T))where I is the current at time t, I.sub.max is the steady-state current after the build-up period, e is the base of the natural logarithm and T is the time constant of the target.
Such a changing current generates a magnetic field, referred to as a secondary magnetic field, in metal detector terminology.
The secondary magnetic field induces a voltage in the receiver coil of a metal detector, and owing to the fact that the time derivative of an exponential function is an exponential function, the resultant voltage has the same time constant as the eddy current, as shown by trace 324, in FIG. 3-C. Owing to its origin, this voltage is referred to as the “eddy-current voltage”.
The eddy-current voltage changes according to the formula:E=E.sub.0×e.sup.(−t/T)where E is the voltage at time t and E sub.0 is the initial voltage.
It is important to note here, that the eddy-current voltage approaches zero, asymptotically. Thus, if the current ramp in FIG. 3-A is long enough, the eddy-current voltage is substantially zero at the end of the ramp.
While the steady-state eddy current does not induce a voltage in the receiver coil, it continues to have an effect on the mutual inductance of the coil system.
In prior-art detectors, a gross reduction of the inductive coupling between the transmitter and receiver coils is usually accomplished by mechanical means, and any residual signal is removed by adding a compensating signal of the appropriate magnitude and polarity to the preamplifier input.
When an object having a higher magnetic permeability than air is brought into the vicinity of such a balanced coil system, the balance is upset, and a voltage is generated in the receiver coil. Such a voltage is termed “mutual-inductance voltage”.
A steady current flowing in a conductive object near the coil system produces an analogous mutual-inductance voltage, but of the opposite polarity.
When a target having both magnetic and conductive properties is brought into the vicinity of the coil system, the signals produced are antagonistic, and the difference between them will be manifested.
During the build-up phase of the eddy current, the eddy-current signals and the mutual-inductance signals are subtracted algebraically. As the eddy-current signal decays, the mutual-inductance signal becomes predominant, as show by traces 408 or 410, in FIG. 4-C.
When a steady state is attained near the end of the current ramp, the residual signal represents the difference between the magnetic imbalance and the effect caused by a steady-state eddy current in the target.
It has been observed that in most targets that are both magnetic and conductive, the magnetic effect predominates, making it possible to determine the magnetic property of the target simply by noting the polarity of the mutual-inductance signal.
This condition is illustrated by trace 410, in FIG. 4-C.
In contrast, a non ferrous target generates signals that are additive during the eddy-current build-up period, as shown by trace 408, in FIG. 4-C
After the build-up effects have subsided, the residual mutual-inductance signal has the opposite polarity of that caused by a ferrous target.
In ferrous targets that sustain large eddy currents, owing to a shape that represents a large area perpendicular to the magnetic flux from the transmitter coil, the sum of the mutual inductance signals may be of indeterminate polarity.
It is therefore advantageous to remove the signal component that represents the steady-state eddy current.
The signal present during the constant-current interval 304, in FIG. 3-A, is a measure of the steady-state current, and it can be sampled there, while the mutual-inductance signal is absent. Sampling this signal at interval 315, shown in FIG. 3-C, and subtracting the value from the sample at interval 328, will make the magnetic imbalance signal more prominent.
This technique will also eliminate another artifact which is termed the “dynamic imbalance signal”.
While the coil system is in motion, relative to magnetic material in the vicinity, the changing mutual inductance between the transmitter and receiver coils will generate a transitory magnetic imbalance signal. Additionally, the realignment of the domains will generate a transitory resistive signal, owing to absorption or release of energy. These signals have essentially the same magnitude at the end of the current ramp and during the constant-current interval, and they can thus be substantially eliminated by the above-mentioned sampling and subtraction.
The signals caused by energy absorption and release can be significantly reduced in amplitude if the coil current is made unipolar, in contrast to the current practice of using bipolar coil excitation. Using a biphasic coil current, as shown in FIG. 4-D, has the advantage of using less energy for a given length of the current ramp, but at the cost of more interference by ground signals.
The choice between the unipolar and biphasic options depends on the application of the detector.
When energy consumption of the detector is of importance, as in battery-powered detectors, extending the current ramp long enough to allow transitory signals to decay to substantially zero, may be impractical.
There is an alternate method of determining the polarity of the of the mutual-inductance signal, however.
In FIG. 4-C, the eddy-current signals 408 and 340 should have the same time constant when calculated from the samples taken at intervals 326, 328 and 342 and 344, respectively. When the signal amplitudes are assigned the designations V1, V2, V3 and V4, in sequence, the time constants can be calculated according to the equation:T1=t/log(V1/V2) and T2=t/(log(V3/V4)where T1 is the time constant during the ramp and T2 is the time constant after the coil pulse. The time difference between the sampling pulses=t, and log is the natural logarithm.
Assuming that there is no significant eddy-current carry-over from the ramp interval to the after-pulse interval, by making the constant-current interval 304 appropriately long, the two time constants should be essentially the same.
It will be found however, that the two time constants may differ significantly, and this can be attributed to the influence of the mutual-inductance signal.
The means to compute and compare the time constants of the signals intercepted by the receiver coil involves the following steps:
Microcontroller 811 directs gating circuit 808 in FIG. 8 to sample the received signals at gating intervals 326 and 328, shown in FIG. 4-C. An algorithm in the program of the microcontroller computes a first time constant T1 from the acquired values per the equation shown in paragraph
The result is stored in the memory of the microcontroller. A second time constant T2 is derived in an analogous fashion from samples 342 and 344, in FIG. 4-C. T2 is then subtracted from T1. If the result of the subtraction is negative, the target is ferrous and if the result is positive, the target is non-ferrous. The actual polarity of the subtraction result depends on how the coils are wired, however, ferrous and non-ferrous targets always yield opposite polarities as a result of the above subtraction.
This determination of the ferrous or non-ferrous nature in unambiguous and it does not depend on stored signatures of targets nor does the method fail, when the time constant of the target signal is similar to the time constant of the background signal owing to mineralization of the soil.
Magnetite, or “black sand”, in the parlance of prospectors, is often found in areas where gold is found. Having a higher specific gravity than the country rock, magnetite is concentrated in the same gravity traps that catch gold nuggets. This circumstance constitutes a major problem for state-of-the-art detectors, since it affects the balance of the coil system.
When only one target is present in the vicinity of the coil system, the demodulated envelopes of the signals of eddy-current origin and those of mutual-inductance origin are essentially synchronous, as shown by traces 508, 604 and 606, in FIGS. 6-A and 6-D. Trace 606 is the response to a ferrous target and trace 604, to a non-ferrous target.
Occasionally, more than one target is detected at the same time, causing state-of-the-art detectors to misidentify a non-ferrous target, when a ferrous target is also present.
This problem is eliminated when the signal envelopes of the eddy-current and mutual-inductance signals are compared with respect to shape and the point in time at which the signals peak.
As shown in FIG. 5-F, the zero-crossing points of the differentiated signals will differ, when two targets with differing magnetic characteristics are present. The rising edges of traces 514 and 520 define the points in time when the two signals reach their peak amplitudes. The difference in the times is used to define the length of “misalignment pulse” 522. When a preset limit of the length of pulse 522 is exceeded, that is taken as an indication of the presence of more than one target, and a ferrous indication, which might normally be issued, is disabled.
The above criterion for multiple-target detection is shown by way of example only. Other criteria may also be used to measure the tracking between the eddy-current and mutual-inductance signals by programming the appropriate algorithm into the microcontroller.
In light of what has been discussed hereinabove, and from what can be seen in FIGS. 3-A, B and C, it can be concluded that by using the appropriate coil energizing current waveform, the various signals intercepted by the receiver coil can be temporally separated. The amplitudes of the various signal components can be ascertained by sampling the signals at appropriate times.
Thus, the use of complicated and ineffective signal-processing means are obviated.